Ribbed slab foundation for cylindrical refrigerated tanks for liquified gas storage

ABSTRACT

A foundation for cylindrical refrigerated tanks for liquified gas storage, at locations where the minimum ambient temperature is always greater than 0° C., characterized by a reinforced concrete ribbed slab structure at grade level, where the clear spaces in between the parallel webs of the ribbed slab are configured as air circulation channels to provide ambient air circulation suitable to prevent the ground underneath the foundation itself from reaching freezing temperatures, i.e. ≤0° C., while providing the necessary bearing and structural capacity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International ApplicationNo. PCT/IB2021/061550 filed Dec. 10, 2021, which designated the U.S. andclaims priority to IT 102020000030440 filed Dec. 10, 2020, the entirecontents of each incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention refers to the Oil & Gas Refining Industry and inparticular to an innovative solution for the foundations of cylindricalrefrigerated tanks for liquified gas storage, such as Propane, Butaneand Ethylene at locations where the minimum ambient temperature isalways above freezing (>0° C.), particularly but not exclusively inTropical and Subtropical Regions.

Description of the Related Art

The internal temperature of this type of tanks, that operate at ambientpressure, is normally and continuously well lower than 0° C. and hencealso the foundation soil might be subjected to temperatures below 0° C.whereas the outside temperature is always higher than 0° C. with highhumidity.

The main issue with these low temperatures is that they would cause soilfreezing and consequent frost heaving and hence the uneven displacementof foundation with damages to either the tank and the piping connectedthereto.

Furthermore, this phenomenon is progressive, meaning that freezing ofthe shallower soil moisture layer usually attracts by capillarity morewater that in turn will freeze as well, building-up an ice lens underthe tank foundation centre and consequently increasing the strain in thefoundation up to a level exceeding its structural capacity orserviceability limits.

In order to prevent this phenomenon, the engineering practice as well asthe international reference codes (i.e.: API 625) recommend twoalternative solutions:

-   -   To install electric heaters within the thickness of the disc        foundation slab, or    -   To construct the disc foundation slab elevated above grade, on a        set of columns.

Both these alternative options have a cost impact: the first one for thecapital and operational cost of electric heating, while the second forthe capital cost of constructing the elevated support structure.

SUMMARY OF THE INVENTION

The innovative idea of this invention—for cylindrical tank foundationsto be erected at sites such as in Tropical and Subtropical Regions wherethe minimum ambient temperature is always above freezing (>0°C.)—consists in constructing a ribbed slab foundation where the ambientair circulation in the clear spaces in between the webs of said ribbedslab will prevent the ground underneath the foundation from reachingfreezing temperatures (≤0° C.) due to the contact of tank bottom withthe top of the foundation slab and, at the same time, the webs act asmain structural component of the foundation.

In case of presence of non-bearing soil strata under the foundation,then piles are required, and in this case the ribbed slab foundationwith its “ribs” directly connected onto the piles allows to implement a“bottom-up” construction sequence, that results much quicker, cheaperand less hazardous in terms of safety, than the usual “top-down”industrial practice. The latter in fact requires first to build a 2.5meter high (minimum) embankment, to execute the piles through it, thento cast the slab on its top and finally to dig-out the embankmentbetween the slab and the ground surface, thus resulting is an expensiveand time consuming activity.

This has been accomplished by designing the foundation as a reinforcedconcrete ribbed slab with its webs parallel to the tank diameter andsupported at grade. The clear spaces in between said foundation webs areopen to the ambient in order to ensure a continuous heat transfer fromambient air—which temperature is always higher than 0° C.—into thefoundation supporting the refrigerated tank containing the liquifiedgas, thus preventing the freezing of soil underneath.

In the following description the term “ribbed slab” is meant to indicatea reinforced concrete slab sitting on its webs directly in contact withthe soil, where said clear spaces between webs function as aircirculation channels.

For a better understanding of the invention, a detailed description isgiven hereinafter with reference to the enclosed figures that areprovided as examples, without any limitation implied therein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

FIG. 1 shows in vertical cross section the behavior of a usualtank/foundation/soil system, i.e. without any air circulation channels,and demonstrates that temperatures lower than 0° C. are indeed occurringin the soil underneath.

FIG. 2 shows in vertical cross section the behavior of thetank/foundation/soil system with air circulation channels according tothe invention, and demonstrates that temperatures are higher than 0° C.both in the foundation and in the soil underneath.

FIG. 3 shows in isometric view the behavior of the tank/foundation/soilsystem with air circulation channels as per FIG. 2 , and hencedemonstrates that temperatures are higher than 0° C. both in thefoundation and in the soil underneath.

FIG. 4 shows in vertical view parallel to the channels, the behavior ofthe tank/foundation/soil system with air circulation channels as perFIG. 2 , and confirms that temperatures are higher than 0° C. both inthe foundation and in the soil underneath.

FIG. 5 is a plan view of the foundation showing the parallel webs andthe resulting channels having rectangular cross section.

FIG. 6 is a vertical cross section showing the air circulation channelsin between the foundation webs having rectangular cross section,constructed using removable formworks.

FIG. 7 is a vertical cross section of the reinforcement arrangement inthe cross section of the ribbed slab foundation showing the “ribs” thatsupport the slab while integrating channels for the natural ventilationof the structure.

DETAILED DESCRIPTION

According to the invention, the foundation (FV) is provided with anumber of air circulation channels (C) (hereinafter called “channels”),evenly distributed in plan and preferably parallel each other and to thebottom of the tank, that are passing from one side of the foundation(FV) to the opposite one with a substantially constant cross section,with a nominal longitudinal slope and with both ends open to thesurrounding ambient air.

The ventilation of these channels is preferably, though not exclusively,natural and their minimum transversal cross section dimension is 600 mmto allow their visual inspection.

During the experimental campaign, the real behavior of thetank/foundation/soil (S/FV/T) system has been analyzed by means of afinite elements 3D Model (prepared using the ANSYS software), in orderto simulate the heat transfer from ambient air in the channels and theliquified gas inside the tank (S).

This thermal analysis is required during the design stage of thefoundation (FV) both to select the size and spacing of air circulationchannels (C), and to allow performing the structural design of theribbed slab foundation (FV) that creates these channels, taking intoaccount the reduction of structural cross section due to the presence ofsaid channels, as well as of any piles required to support thefoundation itself.

The ribs and hence the air circulation channels (C) are preferablyrealized by means of formworks removable after concrete hardening. Inorder to enhance the stack effect, it is preferable that said channels(C) are prismatic and have a nominal longitudinal slope, these featureswill also facilitate their inspection and possible cleaning.

According to the invention, the orientation of air circulation channels(C) is preferably perpendicular to the prevailing wind direction at thesite where the described foundation (FV) is placed, with the aim tominimize the possibility of sand or dirt being dragged inside thesechannels.

The next table shows the data relevant to three prototype-foundationsconstructed for “experimental” purposes for three refrigerated tanks (S)with double containment for liquified gas, having the following mainfeatures:

Tank: D-0001 D-0002 D-0003 External 45 46 46 diameter (meters) Internaltank 27 27 27 height (meters) Design −46° C. −7° C. −46° C. TemperatureProduct Propane C3 Butane C4

3/C4 Foundation: Diameter 46 48 48 (meters) Thickness 1.8 1.8 1.8(meters) Air circulation n.12 × n.12 × n.12 × channels(meters) 1.8 × 1.01.8 × 1.0 1.8 × 1.0 Minimum +11° C. +11° C. +11° C. ambient temperatureNote: even if the design temperature of tank D-0002 is −7° C. instead of−46° C., like the two others, its foundation has been designed with thesame features of the others both for construction standardization, andin view of a possible future change of tank contents and accordingly ofdesign temperature.

indicates data missing or illegible when filed

FIG. 1 of the finite element 3D model for a usual tank/foundation/soilsystem without air circulation channels (C) shows clearly thattemperatures lower than 0° C. can be reached in the soil (T) underneaththe foundation (F).

Whereas, FIGS. 2, 3 and 4 of the finite element 3D model of thetank/foundation/soil system with air circulation channels (C) inaccordance with the present invention, show clearly that temperatureslower than 0° C. remain confined within the tank (S), while the soiltemperature remains greater than +5° C. with a +11° C. minimum ambientair temperature.

FIGS. 5, 6 and 7 are showing a plan view and partial cross section ofthe foundation (FV) in accordance with the as-built prototype of thepresent invention, where the channels (C) have been realized by means ofremovable formworks.

LEGEND

-   -   S=tank    -   F=usual foundation    -   T=soil or ground    -   FV=ribbed slab foundation    -   C=air circulation channels    -   P=pile    -   W=web of the ribbed slab

1. Foundation for cylindrical refrigerated tanks for liquified gasstorage, at locations where the minimum ambient temperature is alwaysgreater than 0° C., comprising a reinforced concrete ribbed slabstructure at grade level, where the clear spaces in between the parallelwebs of said ribbed slab are configured as air circulation channels toprovide ambient air circulation suitable to prevent the groundunderneath the foundation itself from reaching freezing temperatures,while providing the necessary bearing and structural capacity; whereinsaid air circulation channels are placed within the thickness of saidreinforced concrete ribbed slab structure of the foundation, and whereinthe cross section of said air circulation channels is calculated by amethod for designing the cross section of air circulation channelsprovided within a foundation for cylindrical refrigerated tanks forliquified gas storage, at locations where the minimum ambienttemperature is always greater than 0° C., in order to ensure enoughnatural ventilation to prevent the ground underneath the foundationitself from reaching freezing temperatures, while providing thenecessary bearing and structural capacity, said method includingperforming a thermal analysis of the real behavior of thetank/foundation/soil system by means of a finite elements 3D Model, inorder to simulate the heat transfer from ambient air in the channels tothe liquified gas inside the tank; wherein said thermal analysis isperformed during the design stage of the foundation both to select thesize and spacing of air circulation channels, and to allow performingthe structural design of the ribbed slab foundation that creates thesechannels, taking into account the reduction of structural cross sectiondue to the presence of said channels, as well as of any piles requiredto support the foundation itself, said method being performed to ensureenough natural ventilation notwithstanding the prevailing wind directionat the site where the foundation is placed.
 2. The foundation of claim1, wherein said air circulation channels are open to the outside ambientto allow the continuous heat transfer from ambient air—which temperatureis always greater than 0° C.—to the inside of the foundation supportingthe tank containing the liquified gas, thus preventing the freezing ofsoil underneath the foundation itself.
 3. The foundation of claim 2,wherein said air circulation channels are evenly distributed in plan andare substantially parallel to each other and to the upper face of thefoundation supporting the tank bottom; where said air circulationchannels are crossing the foundation from one side to the opposite one.4. The foundation of claim 2, wherein said air circulation channels havea 600 mm minimum dimension of their cross section to facilitateinspection and cleaning.
 5. The foundation of claim 2, wherein said aircirculation channels are prismatic in section with a nominallongitudinal slope in order to enhance the stack effect.
 6. Thefoundation of claim 2, wherein said air circulation channels areoriented perpendicular to the prevailing wind direction at the sitewhere the foundation is placed, in order minimize the possibility ofsand or dirt being dragged inside said air circulation channels. 7.(canceled)
 8. The foundation of claim 2, wherein, in case foundationwith piles is necessary, the foundation is configured to be built in abottom-up construction sequence: piles followed by ribbed slab; insteadof the top-down construction sequence adopted in the usual industrialpractice for these cases, namely in construction sequence order:embankment above grade, piles, slab, digging-out of embankment material.9. The foundation of claim 3, wherein said air circulation channels havea 600 mm minimum dimension of their cross section to facilitateinspection and cleaning.
 10. The foundation of claim 3, wherein said aircirculation channels are prismatic in section with a nominallongitudinal slope in order to enhance the stack effect.
 11. Thefoundation of claim 4, wherein said air circulation channels areprismatic in section with a nominal longitudinal slope in order toenhance the stack effect.
 12. The foundation of claim 3, wherein saidair circulation channels are oriented perpendicular to the prevailingwind direction at the site where the foundation is placed, in orderminimize the possibility of sand or dirt being dragged inside said aircirculation channels.
 13. The foundation of claim 4, wherein said aircirculation channels are oriented perpendicular to the prevailing winddirection at the site where the foundation is placed, in order minimizethe possibility of sand or dirt being dragged inside said aircirculation channels.
 14. The foundation of claim 5, wherein said aircirculation channels are oriented perpendicular to the prevailing winddirection at the site where the foundation is placed, in order minimizethe possibility of sand or dirt being dragged inside said aircirculation channels.
 15. The foundation of claim 3, wherein, in casefoundation with piles is necessary, the foundation is configured to bebuilt in a bottom-up construction sequence: piles followed by ribbedslab; instead of the top-down construction sequence adopted in the usualindustrial practice for these cases, namely in construction sequenceorder: embankment above grade, piles, slab, digging-out of embankmentmaterial.
 16. The foundation of claim 4, wherein, in case foundationwith piles is necessary, the foundation is configured to be built in abottom-up construction sequence: piles followed by ribbed slab; insteadof the top-down construction sequence adopted in the usual industrialpractice for these cases, namely in construction sequence order:embankment above grade, piles, slab, digging-out of embankment material.17. The foundation of claim 5, wherein, in case foundation with piles isnecessary, the foundation is configured to be built in a bottom-upconstruction sequence: piles followed by ribbed slab; instead of thetop-down construction sequence adopted in the usual industrial practicefor these cases, namely in construction sequence order: embankment abovegrade, piles, slab, digging-out of embankment material.
 18. Thefoundation of claim 6, wherein, in case foundation with piles isnecessary, the foundation is configured to be built in a bottom-upconstruction sequence: piles followed by ribbed slab; instead of thetop-down construction sequence adopted in the usual industrial practicefor these cases, namely in construction sequence order: embankment abovegrade, piles, slab, digging-out of embankment material.
 19. Thefoundation of claim 7, wherein, in case foundation with piles isnecessary, the foundation is configured to be built in a bottom-upconstruction sequence: piles followed by ribbed slab; instead of thetop-down construction sequence adopted in the usual industrial practicefor these cases, namely in construction sequence order: embankment abovegrade, piles, slab, digging-out of embankment material.
 20. Thefoundation of claim 9, wherein said air circulation channels areprismatic in section with a nominal longitudinal slope in order toenhance the stack effect.
 21. A method for designing the cross sectionof air circulation channels provided within a foundation for cylindricalrefrigerated tanks for liquified gas storage, at locations where theminimum ambient temperature is always greater than 0° C., in order toensure enough natural ventilation to prevent the ground underneath thefoundation itself from reaching freezing temperatures, while providingthe necessary bearing and structural capacity, said method including athermal analysis of the real behavior of the tank/foundation/soil systemby means of a finite elements 3D Model, in order to simulate the heattransfer from ambient air in the channels to the liquified gas insidethe tank; wherein said thermal analysis is performed during the designstage of the foundation both to select the size and spacing of aircirculation channels, and to allow performing the structural design ofthe ribbed slab foundation that creates these channels, taking intoaccount the reduction of structural cross section due to the presence ofsaid channels, as well as of any piles required to support thefoundation itself.